1. Field of the Invention
The present invention relates to intraocular lenses or implants designed to be implanted in the capsular sac after the ablation of the crystalline lens suffering from cataract.
2. Description of the Related Art
The ablation of the crystalline lens is most frequently carried out by capsulotomy of the leaf or anterior capsule followed by a phacoemulsification of the crystalline lens and the cleaning of the site and notably the capsular sac in order to eliminate the epithelial cells. However, Elschnig pearls that form from the germinative cells subsist on the equator of the capsular sac after the surgical operation. These cells migrate along the posterior capsule and cause the opacification of the posterior capsule. Because of the opacification of the posterior capsule, also called second cataract, in the three years following the ablation of the crystalline lens and the implantation of an intraocular lens, approximately thirty per cent of patients need a new operation, namely a capsulotomy of the posterior capsule by Nd:YAG laser, with the disadvantage of the creation of communication between the anterior and posterior segments of the eye. Moreover, the opacification of the posterior capsule (PCO) forms the most common complication in cataract surgery.
In general, the intraocular lens comprises an optic portion which provides the optic and notably refractive correction and a haptic portion comprising at least one haptic element and in practice at least two haptic elements which rest in the equatorial zone of the capsular sac, and even the ciliary sulcus, and ensure the positioning of the optic axis of the optic portion substantially coinciding with the optic axis of the eye and of the pupil. When the haptic elements are angled anteriorly, the optic portion is in contact with the posterior capsule.
In a study by Nagata et al. “Optic sharp edge or convexity: comparison of effect on posterior capsular opacification”, Jpn J Opthalmol., Vol. 40; 397-402, 1996, they found that a rounded edge of the posterior face of the optic allows the invasion of the epithelial cells behind the optic while a sharp edge, also called “square” edge, is effective in reducing the opacification of the posterior capsule. In the article “Posterior capsule opacification” by Nishi, J Cataract Refract Surg—Vol. 25, January 1999, the author confirmed that the sharp or square edge of an optic produces a “discontinuity or a sharp bend” in the posterior capsule which is associated with a reduced incidence of opacification of the posterior capsule.
Square-edged intraocular lenses, also called anti-PCO intraocular lenses, currently exist in two types. On the one hand, there are three-part intraocular lenses comprising an optic portion, for example made of flexible material such as an acrylic polymer, and two haptic elements made of polypropylene or another rigid material anchored in the periphery of the optic portion, and, on the other hand, there are intraocular lenses made in one piece and of flexible material or a combination of rigid and flexible material.
In an article entitled “Evaluation of lens epithelial cell migration in vivo at the haptic-optic junction of a one-piece hydrophobic acrylic intraocular lens”, Nixon and Apple report the much higher frequency of opacification of the posterior capsule requiring a capsulotomy with the intraocular lenses in one part made of acrylic compared with those with intraocular lenses in three parts comprising an optic made of acrylic and fitted haptic elements made of polypropylene. One of the problems with one-piece intraocular lenses reported by these authors is the absence of an effective square or sharp edge at the junction of the posterior face of the optic with the posterior face of the haptic elements. The authors recommend a sharp or square edge of 360° on the posterior face of the optic.
It has also been found that the migration of the epithelial cells is preferably carried out by means of the haptic elements. Specifically, during the capsular symphysis, that is to say in the weeks and months following the implantation of the intraocular lens, the leaves of the sac come together at the haptics and the periphery of the optic, trapping the epithelial cells in zones located essentially at the haptic elements of the implant. These cells then migrate toward the centre of the optic following an axial offset between the optic and the haptic elements.
Also known is document WO 01/03610 which describes an intraocular lens made in one piece of rigid material such as PMMA, or of a hydrophobic or hydrophilic flexible material and notably silicone, the acrylic polymers, and even polyHEMA, the haptic portion of which makes an angle of between 5° and 12° directed toward the anterior face, a zone of connection between the optic portion and the haptic elements, a radial extension of the connection zone of which the posterior face is placed on the spherical cap of the posterior face of the optic portion and a step made by an axial offset between the posterior face of the optic portion and the connection zone of the optic element.
But, with such a geometry, the posterior face of the haptic elements cannot be in the continuity of the posterior face of the optic but is necessarily axially offset toward the front.
Also known is document WO 03/039409 which describes an intraocular lens in one piece or several pieces made of a flexible or rigid material in which the periphery of the optic portion comprises two or three steps from the periphery of the posterior face of the optic portion, in which each of these steps forms a sharp or square edge designed to prevent the migration of the epithelial cells behind the optic portion.
Such a geometry does not provide a solution to the problem of migration at the junction between the haptic elements and the optic portion when the lens is made in a single piece. Similarly, the formation of the plurality of steps in the cylindrical periphery of the optic portion cannot provide a good contact with the posterior capsule in order to form an effective barrier against the progression of the epithelial cells between the posterior face of the optic portion and the posterior capsule.
Finally, document WO 2005/055875 is known in which is described an intraocular lens for implantation into the capsular sac, comprising an optic portion and a haptic portion, the posterior face of the optic portion having a sharp ridge on the peripheral edge and the posterior face of the haptic elements comprising at least one tooth. However, the sharp ridge on the peripheral edge is interrupted at the junction of the haptic and optic elements.